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Neuropathic pain differs from other types of pains. And in order to start talking about neuropathic pain, it's important to remember what pain actually is.
So, nociceptive pain, or pain that we sense usually, is nerves transmitting an impulse and letting us know that there is a damage or injury to a part of our body. Neuropathic pain is an actual pathology of the nerve itself.
Nerve consists of its body, which is the axon, which is the part that usually gets injured. And when that axon is injured, what happens is abnormal transmission of the impulses. It's important to remember that it's not nerves communicating an injury elsewhere, but the process affecting the nerves themselves.
Neuropathic pain itself presents differently from other types of pain. So, neuropathic pain is likely to be severe. It is usually sharp. It is electric, shock-like sensation that most people describe. It is lightning or lancinating type of pain that most people talk about when they describe neuropathic type of pain. Accompanying that, it can be a deep burning or, at the same time, it can also present as coldness in the limbs or distribution of that nerve. It also comes, at times, with persistent numbness, tingling, or weakness of the muscles that that nerve supplies.
Neuropathic pain usually travels along the path of the nerve itself. Because the nerves have different function—some nerves are motor nerves, some nerves are sensory nerves—if the sensory part of the nerve is affected, then it can alter sensation. Now, it can actually decrease sensation—in other words, create numbness—or it can heighten sensation, where normal stimuli are now painful or altered. So something that would usually be a normal muscle sensation, such as light touch, can become a painful sensation.
There are many causes for neuropathy, or neuropathic pain, affecting the nerves. Some of those are compression of the nerve. Now, that compression of the nerve can occur anywhere along the path of the nerve. It can be as it exits the spine and travels onward, as in radiculopathy, or in other words, pain arising from compression of the spinal nerve before it exits the spinal cord, or the spinal column, rather. Or it can be peripheral nerve compression. And many of us know what it feels like when we cross our legs and the leg goes numb; that is compression of the peripheral nerve, and usually that recovers by itself. But if that compression remains for a longer period of time, then that can become, not necessarily permanent, but the recovery from that can take months and sometimes even a whole year.
Other sources of the neuropathy can be systemic processes, such as diabetes. Diabetes is a microvascular process decreasing the supply of nutrients to tissues as well as nerves and that tends to be what's called a length-dependent process. In other words, nerves that are longer tend to be affected first. And that's why people with diabetic neuropathy tend to feel their feet being affected first. Because those nerves are that much longer, so those nerves tend to be affected first, then the hands follow that because they are the next-longest nerves in the body.
That being said, any process that can damage tissue can also damage nerve tissue. So, what I mean is, treatments like chemotherapy, there are different chemotherapy agents that can alter different processes that the nerves depend on and some are actually neurotoxic. In other words, they are damaging the nerves directly. Now, chemotherapy affecting the nerves can appear at the time of the treatment, but it can also be a delayed presentation of that neuropathy. Also, radiation. A process that radiation causes, in the long run, can come up as post-radiation fibrosis. In other words, tissues fibrose and contract and can affect the nerves, and that can cause neuropathy as well.